The present invention relates to a heat resistant masking material.
In general, a pressure-sensitive adhesive tape having a pressure-sensitive adhesive layer containing an acrylic polymer as a main component on backing layer is known. Because the acrylic pressure-sensitive adhesive is generally superior in weatherability. Particularly, in case the acrylic pressure-sensitive adhesive is crosslinked, it becomes possible to be provided with the heat resistance. An example of the crosslink-type acrylic adhesive is disclosed in U.S. Pat. No. 3,284,423. This crosslink-type acrylic adhesive contains (a) 35 to 75% by weight of an acrylate ester having 6 to 15 carbon atoms; (b) 10 to 60% by weight of methyl acrylate or ethyl acrylate; (c) 0.1 to 10% by weight of an acid component such as (meth)acrylic acid, itaconic acid, or crotonic acid; and (d) 0.1 to 10% by weight of glycidyl (meth)acrylate, and is self-crosslinked at room temperature or upon heating. As a result, the crosslink-type acrylic pressure-sensitive adhesive can have both a cohesive force and a holding force and a sufficiently high adhesive force at high temperature. Preferably, glycidyl (meth)acrylate is present in an amount of 1 to 3% by weight, thereby to impart a desired cohesive force to the above crosslink-type pressure-sensitive acrylic adhesive. This type of pressure-sensitive adhesive can maintain a high adhesive force even after cross-linking by using methyl acrylate or ethyl acrylate as a monomer component.
Further, Japanese Patent No. 2955095 discloses a pressure-sensitive adhesive for a surface protecting film, comprising a copolymer derived from copolymerizing (meth)acrylate ester monomer with a carboxylic group-containing copolymerizable monomer, the copolymer being cross-linked by an epoxy compound having two or more epoxy groups such as a polyglycidyl ether or polyglycidyl amine, wherein the pressure-sensitive adhesive after curing has a 10% modulus of 0.8 to 4.0 kgf/cm2. It describes that the pressure-sensitive adhesive is used for protecting a surface of a resinous board. It further describes that it allows a high-speed release of the protecting film with the pressure-sensitive adhesive from the resinous board, by adjusting the modulus of adhesive to 0.8 kg/cm2 or higher. However, it is very difficult to have two opposed properties of high initial adhesive strength and adhesive strength during use, and cohesive force enabling release of the adhesive after being used.
On the other hand, a pressure-sensitive adhesive is proposed, which is capable of providing a sufficient adhesion at time of use and capable of lowering its adhesion after use, whereby easily released or removed. Japanese Unexamined Patent Publication (Kokai) No. 10-25456 discloses a pressure-sensitive adhesive sheet, which can easily be released and removed upon heating. The pressure-sensitive adhesive disclosed in Japanese Unexamined Patent Publication (Kokai) No. 10-25456 generally comprises a pressure-sensitive adhesive layer containing 100 parts by weight of a tacky base polymer, 10 to 900 parts by weight of a thermosetting compound and 0.1 to 10 parts by weight of a thermal polymerization initiator. The base polymer of the pressure-sensitive adhesive sheet enables strong adhesion at the time of use. However, when the pressure-sensitive adhesive sheet is once heat-treated after use, the adhesive force is lowered and it becomes possible to easily release the pressure-sensitive adhesive sheet. The thermosetting compound is three-dimensionally crosslinked by heating to a temperature of 30 to 150° C. in the presence of the thermal polymerization initiator such as organic peroxide and thus lowering the adhesion of the pressure-sensitive adhesive sheet. Although a low-molecular compound or oligomer having at least two carbon-carbon double bonds is commonly used as the thermosetting compound, such a low-molecular compound or oligomer is likely to remain in the pressure-sensitive adhesive layer without being crosslinked, resulting in a residual of adhesive upon releasing. Further, since it is not cross-linked at time of use, the cohesive force is low and thus it cannot be repositioned. It is more likely to lower a cohesive force of the adhesive, resulting in a residual of adhesive, if the adhesive encounters severe conditions such as a plasma treatment.
In the field of masking tape etc. for lead frame used such as in manufacturing of chip-scale packaging, a pressure-sensitive adhesive tape capable of resisting the increasingly severe conditions is required. For example, a pressure-sensitive adhesive tape as a masking tape is required which has a sufficient initial adhesion to an adherent and a cohesive force for repositionability, has a stable adhesive strength at time of heat treatment at a high temperature for a long time and a plasma treatment, and later, can be easily released without a residual of adhesive.